At the same time, coreference resolution using the ON scheme as a target has been plagued by a number of issues: the lack of annotation of singletons (entities mentioned only once) has led to systems conflating referentiality recognition (whether an expression in fact refers to some entity) with coreference recognition (whether a referring expression is mentioned more than once, see [1]}, [2]}); omission of annotation of indefinite anaphors has led to counterintuitive system behaviors (e.g. repeated mention of specific indefinites and generics is ignored; see Section REF  and the Appendix for examples); and idiosyncratic handling of some constructions (especially copula predication, compounding) has meant that even when systems perform well in predicting ON-like annotations, the results are often less informative than they could be for practical applications (see Section REF ). Additionally, because the ON guidelines for English depend on morphosyntactic constructs such as indefinite NPs, copula predication and compounding, they are not readily applicable to other languages, leading to rather different practices in corpora for languages other than English, including in the Arabic and Chinese sections of the OntoNotes corpus itself.
